1. Field of the Invention
The present invention generally relates to a scintillation camera apparatus and a method capable of eliminating from a gamma-ray spectral energy distribution signal, a scattering signal component of gamma rays emitted from a radioisotope such as .sup.99m Tc (technetium) which has been injected into a biological body under medical examination. More specifically, the present invention is directed to such a method and a gamma camera apparatus that the gamma-ray scattering signal components caused by a scattering phenomenon occurring within the biological body and also the gamma camera apparatus can be quantitatively removed from the gamma-ray distribution signal by way of a multiple window setting method.
2. Description of the Prior Art
In nuclear medical diagnostic systems, a radioisotope is injected into a biological body under medical examination, and a distribution of the injected radioisotope is imaged by a gamma camera for a medical diagnostic purpose. To improve diagnosing capabilities by such nuclear medical diagnostic systems, .gamma.(gamma)-ray scattering signal components must be effectively removed from the two-dimensional image data indicative of the .gamma.-ray energy distribution within the biological body imaged by the gamma camera. These .gamma.-ray scattering signal components are mainly caused by the gamma-ray scattering phenomenon occurring within the biological body and also within the gamma camera (for instance, an internal portion of a collimator and an NaI scintillator).
Conventionally, the following two methods have been proposed in order to eliminate the above-explained .gamma.-ray scattering components from the two-dimensional image data (indicative the .gamma.-ray energy distribution) acquired by the gamma camera.
In accordance with the first .gamma.-ray scattering component removing method, as represented in FIG. 1, one main window "Wm" is set around a photopeak of a .gamma.-ray energy spectrum curve and also another subwindow "Ws" having a width similar to that of the main window "Wm" is set on a lower energy-level portion of this .gamma.-ray energy spectrum curve with respect to the energy level of the photopeak.
Assuming now that .gamma.-ray image data acquired from an energy region defined by the main window "Wm" is M(x, y) (symbols "x" and "y" indicate two-dimensional coordinate system), and .gamma.-ray image data acquired from an every region defined by the subwindow "Ws" is S(x, y), desirable .gamma.-ray image data C(x, y) from which the scattering signal components have been removed may be calculated from the below-mentioned equation (1). EQU C(x, y)=M(x, y)-R.times.S(x, y) (1)
where symbol "R" indicates a preselected constant.
The above-described first conventional scattering-component removing method is known from, for instance, "Improved SPECT Quantification Using Compensation for Scattered Photons" written by Ronald J. Jaszczak et al., BASIC SCIENCES, The Journal of Nuclear Medicine, volume 25, No. 8, pages 893-900, 1984.
Also, in accordance with the second conventional scattering component removing method, as indicated in FIG. 2, "N" (N is greater than 1) pieces of .gamma.-ray image data acquired by utilizing a window "W.sub.EN ". This window "W.sub.EN " owns a narrow width ".DELTA.E" capable of sufficiently reproducing a .gamma.-ray spectral energy distribution curve. As a result, a shape of this .gamma.-ray spectral energy distribution curve can be grasped, scattering signal components are inferred from this distribution curve shade, and then the inferred scattering signal components are removed from the image data on the photopeak, whereby desirable .gamma.-ray image data can be obtained.
The second conventional .gamma.-ray scattering-component removing method is known from, for example, "SPECT Compton-Scattering Correction by Analysis of Energy Spectra" written by Kenneth F. Koral et al, BASIC SCIENCES, The Journal of Nuclear Medicine, volume 29, No. 2, pages 195-202, 1988.
However, the first conventional .gamma.-ray scattering-component removing method has the following drawbacks. That is, although the .gamma.-ray image data in the energy range defined by the main window "Wm" and containing the photopeak is wanted to be correctly acquired, this energy range contains the .gamma.-ray scattering signal components. Nevertheless, actual .gamma.-ray scattering-components contained in other energy range as defined by the subwindow "Ws" are measured. Then, based on this actually-measured scattering components, the first-mentioned scattering components contained in the desirable .gamma.-ray image data with the photopeak are inferred. As a consequence, the first-mentioned scattering components are not correctly or actually measured, and thus the desirable .gamma.-ray image data from which the actual scattering components have been eliminated cannot be obtained.
In accordance with the second conventional .gamma.-ray scattering-component removing method, precision of the scattering-component elimination may be improved as compared with the first conventional removing method. However, since a large quantity of image data must be acquired by utilizing the window having such a very narrower width ".DELTA.E" than that of the main or subwindow "Wm", "Ws", a lengthy time period is necessarily required so as to entirely acquire such .gamma.-ray image data. Moreover, when other .gamma.-ray scattering components of a specific radioisotope having more than two photopeaks are measured, plenty of time is required to acquire entire image data, resulting in practical difficulties. In other words, real-time .gamma.-ray imaging operation can be hardly realized.